Re: [PATCH v3 09/11] dm: support IO polling for bio-based dm device

On 2/9/21 4:07 PM, Ming Lei wrote:

On Tue, Feb 09, 2021 at 02:13:38PM +0800, JeffleXu wrote:

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On 2/9/21 11:11 AM, Ming Lei wrote:

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On Mon, Feb 08, 2021 at 04:52:41PM +0800, Jeffle Xu wrote:

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DM will iterate and poll all polling hardware queues of all target mq
devices when polling IO for dm device. To mitigate the race introduced
by iterating all target hw queues, a per-hw-queue flag is maintained

What is the per-hw-queue flag?

Sorry I forgot to update the commit message as the implementation
changed. Actually this mechanism is implemented by patch 10 of this
patch set.

It is hard to associate patch 10's spin_trylock() with per-hw-queue
flag. 

You're right, the commit message here is totally a mistake. Actually I
had ever implemented a per-hw-queue flag, such as

struct blk_mq_hw_ctx {
	atomic_t busy;
	...
};

In this case, the skipping mechanism is implemented in block layer.


But later I refactor the code and move the implementation to the device
driver layer as described by patch 10, while forgetting to update the
commit message. The reason why I implement it in device driver layer is
that, the competition actually stems from the underlying device driver
(e.g., nvme driver), as described in the following snippet.

nvme_poll
	spin_lock(&nvmeq->cq_poll_lock);
	found = nvme_process_cq(nvmeq);
	spin_unlock(&nvmeq->cq_poll_lock);

It is @nvmeq->cq_poll_lock, i.e., the implementation of the underlying
device driver that has caused the competition. Thus maybe it is
reasonable to handle the competition issue in the device driver layer?

Also scsi's poll implementation is in-progress, and scsi's poll may
not be implemented in this way.

Yes. The defect of leaving the competition issue to the device driver
layer is that, every device driver supporting polling need to be somehow
optimized individually. Actually I have not taken a close look at the
other two types of nvme driver (drivers/nvme/host/tcp.c and
drivers/nvme/host/rdma.c), which also support polling.

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to indicate whether this polling hw queue currently being polled on or
not. Every polling hw queue is exclusive to one polling instance, i.e.,
the polling instance will skip this polling hw queue if this hw queue
currently is being polled by another polling instance, and start
polling on the next hw queue.

Not see such skip in dm_poll_one_dev() in which
queue_for_each_poll_hw_ctx() is called directly for polling all POLL
hctxs of the request queue, so can you explain it a bit more about this
skip mechanism?

It is implemented as patch 10 of this patch set. When spin_trylock()
fails, the polling instance will return immediately, instead of busy
waiting.

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Even though such skipping is implemented, not sure if good performance
can be reached because hctx poll may be done in ping-pong style
among several CPUs. But blk-mq hctx is supposed to have its cpu affinities.

Yes, the mechanism of iterating all hw queues can make the competition
worse.

If every underlying data device has **only** one polling hw queue, then
this ping-pong style polling still exist, even when we implement split
bio tracking mechanism, i.e., acquiring the specific hw queue the bio
enqueued into. Because multiple polling instance has to compete for the
only polling hw queue.

But if multiple polling hw queues per device are reserved for multiple
polling instances, (e.g., every underlying data device has 3 polling hw
queues when there are 3 polling instances), just as what we practice on
mq polling, then the current implementation of iterating all hw queues
will indeed works in a ping-pong style, while this issue shall not exist
when accurate split bio tracking mechanism could be implemented.

In reality it could be possible to have one hw queue for each numa node.

And you may re-use blk_mq_map_queue() for getting the proper hw queue for poll.

Thanks. But the optimization I proposed in [1] may not works well when
the IO submitting process migrates to another CPU halfway. I mean, the
process has submitted several split bios, and then it migrates to
another CPU and moves on submitting the left split bios.

[1]
https://lore.kernel.org/io-uring/20210208085243.82367-1-jefflexu@linux.alibaba.com/T/#m0d9a0e55e11874a70c6a3491f191289df72a84f8 (local)

-- 
Thanks,
Jeffle